RU2375734C2 - Automatically darkening filter with displaced polarisers - Google Patents

Abstract

FIELD: physics; optics. ^ SUBSTANCE: invention relates to optical devices. The structure 10 of a protective automatically darkening filter has two weakly twisted liquid crystal cells 26, 30, inserted in the space between three displaced polarisers 24, 28, 32. The first, second and third polarisers have interelated directions of polarisation. Direction of polarisation of at least one polariser, first or third is displaced from axis which is normal to the direction of polarisation of the second polariser. ^ EFFECT: structure increases uniformity of darkening of the filter in a wide angle of vision of the user, and also improves visibility through the filter owing to reduced oscillation of darkening. ^ 4 cl, 4 dwg

Description

The present invention relates to automatically darkening liquid crystal protective shields or filters that can be used in welding helmets to protect the welder from the light emitted by the welding torch.

State of the art

Automatically darkening liquid crystal protective shields, also known as automatically darkening filters (or ATPs), are often constructed from a combination of polarizing filters and layers of liquid crystals. Examples of such filters are described in US patent No. 6097451 and No. 5825441.

ATPs pass from a light (transparent) state to a dark (almost opaque) state in accordance with the control signal. For applications like welding, where protection against high levels of incident light is required, the filters are usually mounted in the face mask, and the control signal is triggered by the incident light from the welding arc. Incident light illuminates the sensor on the welding helmet, which, in turn, sends a signal to the ATP. Thus, when the filter is not illuminated by the blindingly bright light of the welding arc, it is clean or transparent, but it immediately darkens when illuminated by such a blindingly bright light. This allows the welder to perform welding operations, as well as perform other work outside the welding area, without removing the protective shield.

Conventional filters have one specific drawback, namely: the filtering effect of the liquid crystal layers has an angular dependence. In other words, it may turn out that the area of the filter falling into the user's field of vision darkens unevenly. The spread of the gap between the cells and undesirable birefringence in the adhesive layers or polarizers can also reduce the uniformity of the darkening, resulting in uneven darkening of the visible area of the filter.

SUMMARY OF THE INVENTION

The object of the invention is the design of a protective automatically darkening filter that solves the problem of uneven darkening of its visible area. The design provides the user with more uniform dimming at a wide angle of view in a dark state. The inventive filter contains two weakly twisted optically rotatable liquid crystal cells located between three polarizers. The twist angle of the liquid crystal cell is less than 90 °. Two external polarizers (first and third) have a polarization direction that is offset from the normal axis in the polarization direction of the second (central) polarizer.

One of the embodiments of the claimed invention is a device containing the first, second and third polarizers and the first and second weakly twisted liquid crystal cells, each liquid crystal having a twist angle of less than 90 °. The first weakly swirling liquid crystal cell is located between the first and second polarizers, and the second weakly swirling liquid crystal cell is located between the second and third polarizers. The polarization direction of at least one polarizer, first or third, is offset from the normal axis in the polarization direction of the second polarizer.

The second embodiment of the claimed invention is an automatically darkening filter comprising a welding face shield with a switchable light filter mounted therein. An automatically darkening filter also includes a sensor that detects light incident on it, and a control unit that receives a signal from the sensor about the presence or absence of incident light. In the presence of incident light, the control unit supplies voltage to the switchable filter. The switched filter contains the first, second and third polarizers and the first and second weakly twisted liquid crystal cells with a twist angle of less than 90 °. The first weakly twisted liquid crystal cell is installed between the first and second polarizers. A second weakly twisted liquid crystal cell is mounted between the second and third polarizers. The polarization direction of at least one polarizer, first or third, is offset from the normal axis in the polarization direction of the second polarizer.

The inventive automatically darkening filter differs from the known ones in that at least one of the polarizers, the first or third, is offset from the normal axis in the direction of polarization of the second polarizer. Due to this shift in polarization orientation, better dimming uniformity is achieved. In the case of a shift in the polarization direction of the first or third polarizer, or both of them, only a small amount of light can pass through the filter, causing a softening effect everywhere, causing a decrease in the inhomogeneity of the dimming observed through the filter. Reducing blackout inhomogeneity improves visibility through the filter. Improving visibility is especially beneficial for welders, as they need to see in detail the objects with which they work.

The following is a detailed description of embodiments of the invention and explanatory graphics thereof. Other features of the invention and advantages achieved will also become apparent from the description, drawings and claims.

Brief Description of the Drawings

Figure 1 presents an exploded view of an example of a structure 10 of a protective switchable filter made in accordance with the claimed invention;

figure 2 schematically shows an example of the orientation of the polarizers and liquid crystal cells in the switchable filter 10, shown in figure 1;

figure 3 shows a block diagram of a switched filter 10, shown in figure 1, mounted in an automatically darkening filter 60;

figure 4 shows in the polar coordinate system the effect of filtering light at different angles of view for the design of the switched filter 10, shown in figure 1.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Figure 1 presents an exploded view of an example of a design of a protective switchable filter 10, suitable for use in an automatically darkening filter. The element farthest from the center is the interference filter 22, designed to exclude ultraviolet and infrared radiation and limit the range of transmitted wavelengths. The filter 10 also contains a first polarizing filter 24, a first optically rotatable liquid crystal cell 26, a second polarizing filter 28, a second optically rotatable liquid crystal cell 30 and a third polarizing filter 32. The device may also contain (but this is not necessary) cell 34, conditionally called " -master".

The first and second liquid crystal cells 26 and 30 are weakly twisted liquid crystal cells. The term "slightly twisted" in this document means that the angle of twist is less than 90 °. For example, at least one of the cells 26 or 30 has a twist angle of less than 90 °, typically from zero or 1 ° to 89 °. In particular, the twist angle of the first or second cells 26 or 30 lies in the range from about 20 ° to about 85 °. The first and second liquid crystal cells 26 and 30 are provided with connectors 52 and 54, respectively, by which a control voltage can be applied.

Each of the polarizers, the first 24, second 28 and third 32, have polarization directions correlated with each other, shown in FIG. The term “polarizer” as used herein means a polarizing filter that transmits light waves along one axis and absorbs them along others. Typically, the transmission and absorption axes of polarization are directed at an angle of 90 ° to each other. The polarization axis or polarization direction of the first polarizer 24 and the third polarizer 32 can be characterized with respect to the direction of the second polarizer 28. The second polarizer 28 has a polarization direction 47. Normal 56 is perpendicular to the polarization direction 47 of polarizer 28. In other words, the angle between normal 56 and direction 47 polarization is 90 ° in the embodiment of the invention shown in figure 1. The polarization directions of both the first polarizer 24 and the third polarizer 32 are offset from the normal 56. In other words, the polarization direction 41 of the first polarizer 24 is offset from the normal by an angle of 40 (hereinafter, the first offset angle 40). Similarly, the polarization direction 53 of the third polarizer 32 is offset from the normal 56 by an offset angle 46 (hereinafter, the second offset angle 46). The offset range provided by the first and second offset angles may be 1-20 °. In one embodiment of the invention, angles 40 and 46 are in the range of 2-8 °.

Figure 1 further shows that both polarizers: the first 24 and the third 32, are offset from the normal 56. However, in some embodiments of the invention, only the first polarizer 24 or the third polarizer 32 is offset from the normal 56. For example, in one embodiment, the direction the polarization 41 of the first polarizer 24 is offset from the normal 56, while the polarization direction 53 of the third polarizer 32 is essentially parallel to the normal 56. In another embodiment, the polarization direction 41 of the first polarizer 24 can be substantially parallel to the normal 56, whilehow the polarization direction 53 of the third polarizer 32 is offset from the normal 56. Thus, the polarization direction of at least one of the polarizers: the first 24 or the third 32, is offset from the normal 56.

The first offset angle 40 can be shifted clockwise, i.e. in the positive direction, while the second offset angle 46 can be shifted counterclockwise, i.e. in a negative direction. The offset angles 40 and 46 can reverse directions or, in some embodiments of the invention, they can have the same directions (for example, both can be positive or both negative). The magnitude of the angles 40 and 46 of the displacement may be the same in some cases, in other cases - different.

Liquid crystal cells 26 and 30 are “slightly twisted” cells. This means that they have a twist angle of less than 90 °. A typical design of this type of weakly twisted cells includes a twist-nematic liquid crystal material located between the glass plates. The inner surfaces of the glass plates of the liquid crystal cell are coated with transparent electrically conductive electrode layers (for example, indium and tin oxide layers), on which, for example, a pre-machined layer of polyimide is applied, for example, is brushed or rubbed along a certain direction. The resulting defining surfaces of the liquid crystal cause the nematic molecules to occupy certain angular positions, so that the molecules are twisted by a certain angle between the defining surfaces. In an electrically inactive state (when voltage is not applied), the plane of polarization is rotated as soon as light passes through the cell and the filter becomes transparent. The orientation of nematic liquid crystal molecules can be controlled by applying an electric field between the defining surfaces. Applied voltage creates an electric field between the defining surfaces. Nematic liquid crystal molecules line up mainly in the electric field perpendicular to the defining surfaces, and not parallel to them, and the cell goes into an opaque state. Thus, when a control voltage is applied to weakly twisted cells, a filtering effect is achieved. The rotation angle of nematic molecules can be controlled by a change in the control voltage, and thus the corresponding filtering effect can also be controlled. As a result, the liquid crystal cells 26 and 30 are in a transparent state in the absence of an applied voltage and darken in the presence of an applied voltage.

Cell 34, conventionally referred to as the “host guest,” may include a nematic liquid crystal, whose molecules and atoms can be aligned parallel to the normal 56 in the polarizer 28. The prepared glass surface can facilitate this alignment. An immiscible pigment having ordered anisotropic absorption is a strong absorber in the aligned state. When an electrical voltage is applied, the nematic crystal molecules themselves are located at right angles to the prepared surfaces, causing the pigment molecules to occupy positions in which the minimum amount of light is absorbed. Therefore, the cell 34 provides a filtering effect in the absence of an applied voltage, while the liquid crystal cells 26 and 30 in the absence of an applied voltage transmit light. Thus, the filter 10 provides a certain minimum level of protection and user safety in the event of an unintentional voltage failure.

In the embodiment of the invention shown in FIG. 1, the corresponding alignment directions of the liquid crystal cells 26 and 30 are substantially parallel to each other and are asymmetrically oriented to one another (directed in opposite directions). For example, the alignment direction 45 of the liquid crystal cell 26 is ordered substantially parallel to the direction 49 of the liquid crystal cell 30, but has the opposite direction. Similarly, the alignment direction 43 of the liquid crystal cell 26 is ordered substantially parallel to the direction 51 of the liquid crystal cell 30, but has the opposite direction. This asymmetry of directions is shown in figure 1 by the index arrows.

Figure 2 - schematically shows an example of the location of the first liquid crystal cell 26, the second polarizer 28 and the second liquid crystal cell 30 with respect to each other. Figure 2 shows that the direction of polarization 47 of the second polarizer 28 is perpendicular to the normal 56, as described above. The bisector 58 bisects the angle between the polarization direction 47 and the normal 56. Thus, in this example, the bisector 58 forms a 45 ° angle with the polarization direction 47 and the normal 56. To optimize the quality of operation in the transparent state, the liquid crystal alignment directions 45, 43 and 49, 51 cells 26 and 30, respectively, can be located symmetrically with respect to the bisector 58. For example, if the twist angle of the liquid crystal cells 26 and 30 was equal to 70 °, then the alignment directions 45 and 43 would be symmetrical about 58 relative to the bisector angle of 35 °. Thus, in this example, the angle 57 between the alignment directions 43, 51 and the bisector 58 is 35 °, the same as the angle 59 between the directions 45, 49 and the bisector 58. In another example, the angle between the normal 56 and the directions 43, 51 could be 10 °, and the angle 59 between normal 56 and directions 45, 49 would then be 80 °.

Although a symmetrical orientation is optimal for a transparent state, other orientations can be used, the invention is not limited to that shown in FIG. 1 and FIG. 2 and the example described above. Without going beyond the scope of the invention, other configurations may be used.

FIG. 3 shows a block diagram of an automatically darkening filter (ATP) 60. The automatically darkening filter 60 comprises a switchable filter 10 provided with biased polarizers of the type described in relation to FIG. 1 and FIG. 2. The switchable filter 10 is mounted on the faceplate 66 of the welder that the user wears during welding or in other situations where protection is required provided by the switchable filter 10. The ATF 60 also includes a sensor 64 for detecting light radiation incident on the front surface of the filter 10, for example from the welding arc. The control unit 62 receives a signal from the sensor 64 corresponding to the presence of light radiation or its absence, and generates a corresponding control voltage supplied to the filter 10, thereby controlling the level of dimming provided by the filter 10. When the sensor 64 detects the radiation of the welding arc or other source of light, the control unit 62 may, for example, supply a control voltage to the liquid crystal cells 26 and 30, while relieving the voltage from the host-guest cell 34. This causes the filter to darken, thereby protecting the user from a dazzlingly bright welding arc. In the absence of a welding arc or other light source, the control unit 62 can reduce the voltage applied to cells 26 and 30 or turn it off altogether, and the filter becomes more transparent to the light flux. This allows the welder to perform welding operations, as well as perform work outside the welding zone, without removing the protective shield. In conclusion: the filter design described here provides improved uniformity of illumination of what the user sees in the dark state of the filter at a large viewing angle.

The switchable filter 10, the sensor 64 and the control unit 62 are usually mounted on the frame of the shield 66 of the welding helmet in the form of a removable assembly mounted on the frame directly in front of the eyes of the welder when the helmet is put on. This assembly may be in the form of a rectangular (or other shape) frame or housing, on which a filter 10, a sensor 64, and a control unit 62 are placed. Examples of helmets can be found, for example, in US Pat. Nos. 6185739, 5533206, 5191468, 5140707, 4875235 and 4853973. Clean air may be forced into the under-helmet space of the welding helmet, so the helmet may include a face seal to separate the breathing zone from the surrounding air. Examples of face seals are given in US patent applications Serial Nos. 10/987512, 10/987641, 10/988789, 29/217155, 29/217153, 29/217154, 29/217107, 29/217156.

A measure of the filtering effect is the so-called shading number. The shading number S is related to the light transmittance TL (expressed as a fraction) of the following equation:

S = I + 7/3 × 10 log (1 / TL).

In the filter of the structure described herein, the filtering effect can vary in the range of shading values from about 3.3 in the light state to shading values from about 9 to about 13 in the dark state. The filtering effect can be selected by changing the applied voltage from about 2 V to about 4.5 V.

4 is a polar diagram 80 showing the amount of shading at different angles of view. The data shown in FIG. 4 was obtained by measurement using a switchable filter, the design of which is similar to that shown in FIG. 1 with a first polarizer 24 having a clock angle of 6 °, a third polarizer 32 having a bias angle of 6 °, and liquid crystal cells 26 and 30 having a twist angle of 80 °. The circumference of the polar diagram 80, which is most remote from the center, represents a 30 ° deviation from the perpendicular angle of incidence (marked in the center of the diagram). The diagram was constructed using a step device with a deviation step of 2.5 ° and an azimuth step of 10 °. Areas where each gray level means the same filtering effect were obtained with a shading step of 0.5. The DIN shading scale is indicated by the number 82. Since the twist angle of the liquid crystal cells 26 and 30 differs from 90 ° and since the orientations of the first and third polarizers are offset, the filtering effect in the dark state is more uniform at different viewing angles than in conventional filter designs.

In the measurements, two identical liquid crystal cells were used. In the aforementioned embodiment of the invention, both liquid crystal cells 26 and 30 can be controlled by the same control voltage, and it can be changed to achieve different densities and, thus, obtain a different degree of blackout in the dark state. Such a solution can simplify the electronics used.

However, the liquid crystal cells 26 and 30 need not be identical. In one embodiment of the invention, for example, between a biased polarizer can be placed a liquid crystal cell with a swirl angle of 90 ° and a weakly swirl cell with a swirl angle in the range of 20-85 °. In another embodiment, two weakly twisted cells with different degrees of twist can be located between the offset polarizers. Different cells can be combined so as to achieve an optimal overall effect, depending on the desired end result. For example, it is possible to combine symmetrically and asymmetrically mounted liquid crystal cells, cells with different twisting angles, cells of different thicknesses, etc.

All of the above patents and patent applications, including also those mentioned in the prior art section, are incorporated by reference in this document.

The above are descriptions of various embodiments of the invention. These and other embodiments are not beyond the scope of the claims.

Claims (10)

1. The filter, including the first, second and third polarizers, and the first and second weakly twisted liquid crystal cells, each of which has a twist angle in the range from 1 ° to less than 90 °, while the first weakly twisted liquid crystal cell is located between the first and second polarizers , the second weakly twisted liquid crystal cell is located between the second and third polarizers, the first, second and third polarizers have polarization directions that are related to each other, and the polarization direction, of at least one polarizer of the first or third is offset from an axis normal to the direction of polarization of the second polarizer. 2. The filter according to claim 1, characterized in that the first and second liquid crystal cells are identical, with the same control voltage being applied to the first and second liquid crystals. 3. The filter according to claim 1, characterized in that the polarization direction of the first polarizer is offset from the axis normal to the polarization direction of the second polarizer by the first angle of displacement. 4. The filter according to claim 1, characterized in that the polarization direction of the third polarizer is offset from the axis normal to the polarization direction of the second polarizer by a second angle of displacement. 5. The filter according to claim 1, characterized in that the polarization directions of the first and third polarizers are offset from the axis normal to the polarization direction of the second polarizer. 6. The filter according to claim 1, characterized in that the polarization direction of one of the polarizers, first or third, is shifted clockwise from the axis normal to the polarization direction of the second polarizer, in the direction of the polarization direction of the second polarizer by the first bias angle, and the polarization direction the other of the polarizers, the first or third, is offset from the axis normal to the direction of polarization of the second polarizer, counterclockwise in the direction of polarization of the second polarizer at the second angle of displacement. 7. An automatically darkening filter unit containing the filter according to claim 1, a sensor for detecting incident light from the welding torch and a control unit that, when there is incident light, sends a signal to the filter according to claim 1, causing the filter to become darker. 8. A welding helmet comprising a housing and an automatically darkening filter unit according to claim 7, wherein the automatically darkening filter unit is fixed to the body so that it is located in front of the welder's eyes when the welding helmet is worn. 9. The welding helmet of claim 8, wherein the automatically darkening filter unit is removable. 10. A welding face shield including a switchable light filter mounted on it, a sensor for detecting incident light and a control unit receiving signals from said sensor corresponding to the presence or absence of incident light, and supplying voltage in the presence of incident light to the switch light filter, while being switched the light filter includes a light filter according to claim 1.

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